This invention is concerned with color cathode ray tubes and a tensible shadow mask therefor. A shadow-mask type color cathode ray tube normally comprises an evacuated envelope having a faceplate hermetically sealed to a funnel which terminates in a narrow neck that contains a plural-beam electron gun. The faceplate has on its inner surface a mosaic of discrete deposits of phosphors which emit red, green, and blue light when excited by the respective ones of the three beams of an electron gun. Each beam must fall only upon its assigned discrete phospor deposit. The mechanism that causes the beams to fall only upon the assigned targets is the "shadow mask," which is also known as a "color selection electrode." The mask is located between the beam source and the faceplate and spaced a predetermined distance from the faceplate, a spacing commonly termed the "Q-distance." The mask has a pattern of apertures which may be in the form of circular holes or elongated slots. The apertures are so arranged as to allow the respective beams to fall only on the assigned deposits. The mask does this by blocking out, or "shadowing," unassigned ones of the beams. Thus the "red" beam will fall only upon the red-light-emitting phosphor deposits, the "blue" beam only upon the blue-light-emitting deposits, etc.
The conventional shadow mask is typically a domed sheet of cold-rolled steel. The curves of the dome conform generally to the similarly domed contour of the closely associated faceplate. The mask assembly is usually supported by three or more springs which attach the mask to a skirt on the faceplate by means of studs extending from the skirt. The support means is designed so that the mask can be repeatedly removed and re-installed in exact prior relationship--a facility required by the faceplate screening process. Masks of this type have a rigid frame that helps to maintain their contour, and provides for suspending the mask by the spring means described. Such masks are largely self-supporting in that the contour of the mask is maintained by the domed configuration and the relative thickness of the mask material--typically in the range of 5 to 10 mils. A grave disadvantage inherent in such a configuration is the tendency of the mask to buckle or bulge in "high-light" areas of the picture. The intensity of the electron beams in high light areas can cause intense heating and a localized expansion of the mask material. The heated portion of the mask is thus displaced from its original position and the color purity of the image is degraded.
In conventional shadow mask systems, which are not under tension, localized doming and consequent color degradation can be reduced by using a thicker mask material. Doming can also be prevented by using a very thin mask material--two mils or less--with the mask under tension. When a thin mask, also known as a "foil mask" is under tension, it will not dome or otherwise deform until heating under electron beam bombardment reaches a critical point. The electronic circuits are designed however so that the critical point is never reached. There have been a number of disclosures of tensed foil masks and means for applying and maintaining mask tension. Typical of these is the disclosure of Law in U.S. Pat. No. 2,625,734, which addresses the construction of a taut, planar, foraminous mask, and the mounting of the mask and target (the faceplate) as a unitary assembly within the envelope. The thin metal is clamped in a frame, and the mask is heated and placed under screw tension. Upon cooling, the metal contracts and is thus rendered taut and held in tension by the frame.
In Pat. No. 1,163,495(GB), a multitude of narrow tabs preferably a fraction of a millimeter wide are shown as extending from a thin metal sheet intended as a shadow mask. The object is to provide means whereby such a thin sheet can be fixed in place without the need for a heavy frame. The sheet is placed under tension and the tabs are said to maintain a constant, uniform tension. The mask is attached directly to the glass of the envelope by the tabs, the ends of which are sealed into the glass.
U.S. Pat. No. 4,069,567 sets forth a method of installing a tensed color selection electrode in a color cathode ray tube. The electrode is held by a holder in a hypertensed state. The electrode and the holder are heated as by an oven while the electrode is tensed. The electrode is heated to a higher temperature than the holder by resistance heating, or RF heating. The electrode is then fixed to the holder and the electrode and holder are cooled to room temperature. The captivated electrode remains in a hypertensed state because its thermal coefficient of expansion is greater than that of the holder.
In U.S. Pat. No. 2,654,940, a wire mesh screen is describedin which the screen is placed over a ring resting on a base, and another, larger ring is placed therearound. The peripheral portion of the screen is thus sandwiched between the two rings to obtain a preliminary tightness of the screen. Final tautness of the screen is achieved by utilizing the difference in the thermal coefficients of expansion of the mask and its mounting.
In U.S. Pat. No. 2,842,696 to Fischer-Colbrie, there is disclosed a color cathode ray tube having an internal ledge in the faceplate skirt for receiving a color selection electrode of the grid wire type. The grid wires are bonded to the ledge in spaced adjacency to the faceplate.
U.S. Pat. No. 3,894,321, of common ownership herewith, discloses a foil mask having a central area of apertures for color selection, and an outer sealing area perforated with a plurality of openings through which frit sealing material can flow. The unperforated area between is said to act as an electron shield.
Other prior art of relevance comprises U.S. Pat. Nos. 2,761,990; 3,440,469; 3,638,063; 3,873,874; and 4,495,437.